The present invention is based on Japanese Patent Application No. 2001-000,933, the entire content of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a fixing device for use in fixing toner on a recording medium in an image forming apparatus such as a copier, printer and the like.
2. Description of the Related Art
In conventional image forming apparatuses such as copiers, printers and the like, tone is supplied to a recording medium, and the toner is fixed to the recording medium by a fixing device.
As show in FIG. 1, such a conventional fixing device generally guides a recording medium 1 supplied with a toner t between a pair of fixing rollers so as to fix the toner t to the recording medium 1 between the pair of fixing rollers 2 and 3.
Furthermore, in order to adequately fix the toner t to the recording medium 1, heaters 4 and 5 are provided within each fixing roller 2 and 3, and elastic layers 2a and 3a are provided on the exterior surface of the fixing rollers 2 and 3, so as to heat the fixing rollers 2 and 3 via the heaters 4 and 5, and press together the fixing rollers 2 and 3, thereby increasing the length of the nip part between the elastic layers 2a and 3a provided on the exterior surface of the fixing rollers 2 and 3, such that the toner t is fixed to the recording medium 1 via adequate heat and pressure.
In recent years, however, such image forming apparatuses have been designed for high-speed operation requiring that the toner t is adequately fixed to the recording medium 1 at high speed, and in full color image forming apparatuses, further necessitating that toner t of a plurality of colors is supplied to the recording medium 1 forming a thick layer of toner t, and the thick layer of toner t must be adequately fixed to the recording medium 1.
In such fixing devices, in order to adequately heat fix the toner t to the recording medium 1 at high speed and adequately fix the thick layer of toner t to the recording medium 1, the length of the nip part applying heat and pressure on the toner t must be increased between the fixing rollers 2 and 3.
Methods have been considered wherein the diameter of each fixing roller 2 and 3 is increased, and the elastic layers 2a and 3a provided on the exterior surface of the fixing rollers 2 and 3 are thickened to increase the length of the nip part between the fixing rollers 2 and 3.
When the diameters of the fixing rollers 2 and 3 are increased, however, a problem arises in that the fixing device becomes larger, and the recording medium 1 readily wraps around the fixing roller 2 in contact with the toner t. Further problems arise when the elastic layers 2a and 3a provided on the exterior surface of the fixing rollers 2 and 3 are thickened, in that heat transfer to the exterior surface of the fixing rollers 2 and 3 becomes difficult, such that the toner t cannot be adequately heated, the toner t cannot be adequately fixed to the recording medium 1, and more power is required to adequately heat the exterior surface of the fixing rollers 2 and 3, thereby increasing the running cost of the device.
In recent years, belt fixing devices have been proposed, such as that disclosed in Japanese Laid-Open Patent No. H9-114283, wherein an endless-type fixing belt is looped around a heating roller and a plurality of support rollers, such that the heating roller is pressed against a pressure roller through the fixing belt, and the fixing belt is pressed against the pressure roller via a pressure contact member on the downstream side, such that the nip region between the fixing belt and the pressure roller is lengthened, and the toner is fixed to the recording medium in this nip region.
In the case of such a belt fixing device, however, problems arise in that the fixing device becomes larger because the fixing belt moves while looped around the heating roller and a plurality of support rollers, thereby enlarging the image forming apparatus. Further problems arise inasmuch as the fixing belt cools while is moving looped around the heating roller and a plurality of support rollers, such that toner cannot be adequately heated to be fixed to the recording medium, and in order to adequately fix toner to the recording medium severe heating conditions are necessary for heating the fixing belt via the heating roller, thereby increasing the running cost, and requiring a warm-up time in order to heat the plurality of support rollers via the fixing belt.
Furthermore, in the case of conventional belt fixing devices, the recording medium curls so as to generate wrinkles in the nip part formed by the fixing belt and the roller, and coiling and jamming of the recording medium may occur.
An object of the present invention is to provide a belt fixing device capable of adequately fixing toner to a recording medium without enlarging the device or increasing the running cost.
Another object of the present invention is to provide a belt fixing device which controls curling and wrinkling of the recording medium so as to make coiling and jamming difficult to occur.
The fixing device of the present invention comprises a heating roller having an internal heating is device; a flexible fixing belt wrapped around the exterior side of the heating roller and having a circumferential length longer than the circumferential length of the heating roller; and a pressure roller for pressing against the heating roller through the fixing belt, wherein a slack part of the fixing belt is formed on the downstream side from the pressure contact part between the heating roller and the pressure roller in the direction of transport of the recording medium, such that this slack part touches the exterior surface of the pressure roller.